The Sound of One Rock Falling

Think of it as the extreme-sport version of listening for songbirds: Atmospheric scientists Douglas ReVelle, Rodney Whitaker, and Peter Brown are using microphone arrays to eavesdrop on the baritone rumblings of interplanetary rocks slamming into Earth's atmosphere.

Most of the time nobody sees them hit, but hearing them is much easier. A typical meteor arrives traveling 50 to 300 times the speed of sound, fast enough to create a powerful sonic boom and, if the object is large enough, an explosive flash. These disruptions stir up persistent low-frequency infrasound, similar to the waves from a nuclear test, which can travel thousands of miles without losing significant energy. And infrasound monitoring stations, including four arrays at Los Alamos National Laboratory and a network being built to verify the Comprehensive Test Ban Treaty, already exist to detect such waves anywhere in the world.

A Leonid meteor bursts and sputters over Italy in this false-color computer reconstruction.Photograph courtesy of Antares Amateur Astronomers Group

Combining input from these stations, ReVelle and his colleagues can figure out when and where meteors are striking; the pitch of the waves also indicates the size of the incoming body. The data show that a meteor as powerful as the Hiroshima A-bomb hits Earth once a year, while major fireballs arrive a couple of times a month. Overall, 500 tons of space debris rain down every day. One way or another such information could help avert disaster. "Sooner or later, Earth is going to get hit by a big object; we need to know how many of them are out there," ReVelle says. "And during the Gulf war, there was a big meteor event over the Pacific. Had it occurred over the Gulf, some government could have thought someone was shooting at them."